In the US, a 20 gallon tank is either 2 or 2 1/2 feet long depending on which style you buy. Both of the tanks actually hold less than 20 US gallons of water. As Andywg said, the size in terms of length and width are more meaningful. The good part about using the US gallon designation is that we are all talking about the same tank. If I call something a 30 long, it is a 3 foot tank that is not very tall. If I call it a 29, it is a 2 1/2 ft long tank that holds about the same amount of water but is taller. I don't think either one actually holds much more than 26 US gallons if you have no substrate. The numbers that are on new tanks don't even say gallons on the label, they are used as model numbers in the states. The only place that I know of where they actually measure things in US gallons is in the US. Both of our neighbors use a metric system of measure. They buy fuel, milk etc. in litres.